If you're writing bare-bones javascript for the browser, creating Chrome Apps and Extensions, or using remote coding apps like cloud9, Koding, or Nitrous, you may not need to install Ubuntu. Some tutorials can be done entirely within the browser. The tradeoff is that you won't have a full-featured command line, and you may hit a point where you can't install something that you need.
To start coding within Chrome OS, install Text or Caret as a text editor. (Text stores files in Google Docs and Caret stores the files locally on your machine, which may help you choose.) After that, you're good to go, since Chromebooks come with a browser installed.